Differential receivers are important components of computer networks and other computer interfaces. For example, the Universal Serial Bus (USB), the Low Voltage Differential Signal (LVDS) and the IEEE 1394 (FireWire) Interface all require differential receivers in order to operate. Differential receivers are typically used to distinguish logic states in a received signal, while differential squelch receivers are typically used to distinguish ternary data.
For example, the IEEE 1394 Interface uses differential signaling for transmission and reception of ternary data, namely the logic states "1", "0" and "Z." The logic states "0" and "1" are transmitted as small-swing differential voltages of about .+-.220 millivolts. The "Z" state implies a differential voltage of less than 89 millivolts.
Differential receivers employ differential squelch comparators to distinguish between these voltage levels. To do so, the differential squelch comparator must have a controlled input offset. Generally, an input offset voltage specification is a measure of how much the voltage on one input terminal must differ from the voltage on another input terminal in order to drive the output to the midpoint of its range.
For many analog circuits it is desirable to have an input offset that is as small as possible in order to minimize the noise in the circuit. For differential squelch comparators, a controlled non-zero input offset permits the circuit to discriminate against differential signals below a certain value. This capability is also used in analog-to-digital converters where multiple thresholds are discriminated to quantize an analog signal.
One known technique for creating a controlled input offset is to use a resistor divider network for dividing the input voltage by a specified amount. For example, FIG. 1 shows a conventional resistor divider network 10 which is employed to obtain a precise input referred offset for a differential comparator 12. The output "Z" of the differential comparator 12 is then read by the digital log unit 14. The output "Z" is given as V.sub.out =K*[(PAD-PADN)+(R.sub.1 /R.sub.2)*Vdd], where constant K=A.sub.diff *[1/(1+(R.sub.1 /R.sub.2))] and A.sub.diff is defined as the gain of the differential comparator. FIG. 2 shows a plot of the input referred offset voltage of the conventional differential comparator shown in FIG. 1, wherein the comparator has an input referred offset voltage of 0 v. FIG. 3 shows a plot of the input referred offset voltage of the comparator shown in FIG. 1 with hysteresis, wherein the comparator has an input referred offset voltage of v.sub.t+ when the voltage goes high, i.e., from a negative value to a positive value, and an input referred offset voltage of v.sub.t- when the voltage goes low, i.e., from a positive value to a negative value.
The resistor divider network 10 provides a fixed offset voltage of (R.sub.1 /R.sub.2)*V.sub.dd, which is independent of process and temperature variations. However, employing resistor divider network 10 to create a controlled input offset suffers from several drawbacks. First, to create a small offset requires that the resistors R1 and R2 be mismatched by a very small amount. For example, to create an offset of 89 mV with V=3.0 would require that R.sub.1 /R.sub.2 =0.029. This may be hard to achieve because it requires that R.sub.1 be small compared to R.sub.2. Second, since the offset voltage of the comparator shown in FIG. 1 is determined by the ratio of the resistors of the resistor network, and since this network is fixed once the device is fabricated, the offset can not be changed.
Resistor divider network 10 has a common-mode bias relative to V.sub.dd when the inputs PAD/PADN are not coupled to any external circuit load. This is problematical for interfaces like the IEEE 1394 Interface which relies on the common-mode bias to be V.sub.ss when inputs PAD/PADN are not coupled to another circuit.
If the inputs PAD/PADN are coupled directly to MOS inputs then the center-tap of the termination network between PAD and PADN can be coupled to V.sub.ss via a large pull-down resistor. Thus when the inputs PAD and PADN are not coupled to an external load, the common-mode voltage becomes Vss via the large pull-down resistor. A rail-to-rail comparator has a nonlinear behavior and cannot be used to create a precision input-referred offset at the output.
Another approach to providing a controllable offset in a differential circuit is disclosed in U.S. Pat. No. 4,754,169 to Morris which uses a reference current derived from a reference voltage and an on-chip resistor to set the currents through two input transistors. An offset resistor in the source lead of one of the transistors produces a voltage drop that sets the offset at an input of the differential stage.
In addition to having a controllable offset voltage, differential comparators used in differential receivers must also have other beneficial characteristics. One figure of merit of a differential comparator is how large the differential input voltage must be in order to cause the output voltage to change from high to low, or vice-versa. In particular, when the voltage on the first (non-inverting) input 16 in the differential comparator 12 in FIG. 1 is higher than the voltage on the second (inverting) input 18, then the output Z of the comparator 20 is high. Alternatively, when the voltage on the first input is less than the voltage on the second input, the comparator output is low. The difference between the voltages on the first and second inputs is referred to as the "differential input voltage."
Another figure of merit is referred to as the common mode range (CMR), which is the voltage range over which a small differential input signal can be detected. Most differential comparators have a rather limited CMR as compared to the full power supply voltage range, which is often referred to as the "rail-to-rail" voltage range. Typically, a comparator with p-channel field effect transistor input devices has a CMR from the negative power supply voltages, V.sub.ss (0 volts) to about V.sub.dd -1.5 volts. A comparator with n-channel field effect transistor input devices has a CMR from about 1.5 volts to V.sub.dd.
A relatively large CMR is necessary for many differential receiver applications. For example, the USB specification requires a receiver with a rail-to-rail CMR. The LVDS specification also requires a receiver with a large CMR that conventional comparators are unable to meet. One design that achieves a relatively large CMR range is disclosed in the IEEE Journal of Solid-State Circuits, Vol. 30, February 1995, pp. 156-159. However, that design is relatively complicated and requires a large integrated circuit (IC) chip area since it is more like an operational amplifier than a comparator.
It is therefore an object of the present invention to create a differential squelch receiver having an input offset that can be easily controlled to a specified level. It is another object of the invention to provide a differential squelch comparator having a wide (rail-to-rail) CMR. It is a further object of the invention to create a differential squelch receiver that does not require a resistor divider network to implement a controlled input offset.